Posted: 22/03/2016 at 15:51
Rubi, trees or shrubs reaching 5-6 metres, 17 to 20 feet high can't be classed as small by any stretch of the imagination. In your original post you referred to them being used for screening which suggested that these would be a continuous line.
It seems that you have modified your original thoughts, and that's not a problem at all. It actually helps for making meaningful suggestions for practical solutions.
As for the effect on neighbours gardens. The sun isn't always high in the sky, and even when it is a line of trees (your original idea) would undoubtedly have cast a massive shadow over adjoining gardens. That's academic now as the ideas have changed.
I'd go along with the idea of a crab apple. It gives flower colour in the spring and wonderful autumn colour from the fruit. The fruit isn't poisonous, just tastes horrible, which might actually be a good thing with a young child. They might try it once, but never again!
Rowan (Mountain Ash) is another possibility, again good autumn colour from fruit and leaf. You'd need to check out whether the fruit is poisonous on that one.